1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a power on reset (POR) circuit for a semiconductor device.
2. Description of the Related Art
In general, a POR circuit is necessary for a semiconductor device. Typically, the POR circuit consists of an external resister and a capacitor. However, the current trend is for the POR circuit to be fabricated on the same semiconductor die as the rest of the circuit.
The function of the POR circuit is to prevent a malfunction from occurring when a power supply voltage is applied to a semiconductor device. When the semiconductor device is operated before the power supply voltage reaches a suitable operational level, the semiconductor device can malfunction. Accordingly, a reset signal (RESET) is needed which is xe2x80x98ONxe2x80x99 when the power supply voltage is applied but has not risen to a predetermined voltage level and is xe2x80x98OFFxe2x80x99 after this power supply has reached a stable operating voltage
FIG. 1 shows a conventional POR circuit. A Schmitt Trigger 103 is used for the POR circuit. That is, once the power supply voltage VDD reaches or surpasses a first predetermined voltage threshold after the initial application of the power supply voltage, then the output xe2x80x98Voutxe2x80x99 becomes active, i.e. logic xe2x80x9clowxe2x80x9d, and when the power supply voltage VDD falls below a second predetermined voltage threshold, then the output xe2x80x98Voutxe2x80x99 becomes inactive, i.e. logic xe2x80x9chighxe2x80x9d. The threshold voltage of pull-up transistor 101 determines the value of the first predetermined threshold voltage in the circuit of FIG. 1, which, in turn, causes the output xe2x80x98Voutxe2x80x99 to turn-on during the application of the power supply voltage. The threshold voltage of pull-down transistor 102 determines the value of the second predetermined voltage threshold in the circuit of FIG. 1, which causes the output xe2x80x98Voutxe2x80x99 to turn-off when the power supply voltage is removed. Accordingly, the threshold voltages of the pull-up transistor 101 and the pull-down transistor 102 can be controlled in order to determine the voltage at which operation of the semiconductor device is initiated and the voltage at which operation of the semiconductor device is suspended.
However, the presence of a noise signal in the supply voltage VDD can cause the POR circuit of FIG. 1 to behave abnormally. This is because the input terminal N104 of the Schmitt Trigger 103 does not have a discharge path. As a result, the output terminal xe2x80x98Voutxe2x80x99 may transit to an xe2x80x9cONxe2x80x9d state in response to an unstable state of the power supply voltage. Also, the power supply voltage xe2x80x98VDDxe2x80x99 can be unstable during operation of the semiconductor device and the POR circuit of FIG. 1 is unable to detect this unstable state and reset the semiconductor device. Also, the circuit is highly sensitive to changes in the semiconductor fabrication process and a small change in the process can cause a large change in the first and second voltage thresholds.
It is an object of the present invention to provide a power on reset (POR) circuit which is process independent and which is capable of preventing malfunction in a semiconductor circuit when noise is present in a power supply voltage before the power supply voltage has reached an operational level.
Accordingly, to achieve the above object, there is provided a POR circuit including a differential amplifier for sensing and amplifying a difference in voltages between a non-inverting input terminal and an inverting input terminal, to generate an output signal. The POR circuit further includes a non-inverting input control circuit controlling a voltage of the non-inverting input terminal of the differential amplifier. Also, the POR circuit includes an inverting input control circuit controlling a voltage of the inverting input terminal of the differential amplifier.